The advantages of PSK over AM or FSK have been recognized by the art such that the field of PSK communications is rapidly expanding. Commensurate with the expansion of this field, many varieties of modulation and demodulation have been described in the prior art. These systems have pre-arranged coding and modulation rules to match input information rates with the transmission characteristics of whatever transmission medium is being employed so as to maximize the probability of correct transmission, while, at the same time, minimizing required bandwidth and the effect of noise.
One type of PSK system can be thought of as direct PSK wherein the intelligence to be transmitted is translated into a selected phase with regard to a carrier phase or absolute phase. One difficulty associated with this technique is the requirement that the receiver have information from which it can determine what the carrier phase or absolute phase is at any time so that the received phase may be demodulated to produce the transmitted information.
Another category of PSK systems which does not exhibit this disadvantage is so-called differential PSK. In this arrangement, the information transmitted is encoded as a particular phase with reference to the previously transmitted phase. With this arrangement, the receiver need not have available to it a carrier reference, since the received phase can be demodulated by comparing it with the received phase of the previous modulation level.
Insofar as I am informed, all of these PSK systems, whether they be direct or differential, require two modulation levels for each pair of symbols to be transmitted. Thus, for example, a mark (or 1) will be transmitted by a given phase transition whereas a space (or 0) would be transmitted by a different phase transition. Either one of these phase transitions could, in fact, be no transition.
In order to increase the information capacity of PSK communication systems, arrangements have been provided in the prior art in which multiple modulation levels were available (such as 4). With this scheme, a complex symbol, corresponding to a two bit word, could be transmitted with but a single phase transition. While this increases the rate at which information could be transmitted, nevertheless, in this and other systems, a modulation level was reserved for each symbol transmitted. More particularly, the symbol 01 was transmitted by a particular phase transition or modulation level, whereas the signal 10 was transmitted by a different phase transition or modulation level.
I have found that it is possible to transmit two different symbols with the same phase assignment and thus it is possible using the principles of my invention to provide clocking for each data state and, in addition, a "no-data" state.
It is therefore one object of the present invention to provide a more efficient PSK communication system. It is another object of the present invention to provide a PSK communication system in which two symbols can be transmitted with the same modulation level or phase assignment. It is yet another object of the present invention to achieve the foregoing improvement in efficiency while at the same time not increasing the spectrum bandwidth. It is another object of the present invention to provide a system of the foregoing type which is asynchronous and therefore does not require a clocking signal to be transmitted along with it. It is a further object of the present invention to provide a PSK communication system which is insensitive to data patterns. Finally, it is an object of the invention to meet the foregoing objects with a system exhibiting relative simplicity.